Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session Y34: Surfaces and Interfaces II |
Hide Abstracts |
Sponsoring Units: DCP Chair: Andrew Rappe, University of Pennsylvania Room: LACC 511A |
Friday, March 25, 2005 11:15AM - 11:27AM |
Y34.00001: Density Functional Approach to Investigate the Stability of Nanobubbles Gota Kikugawa, Shu Takagi, Yoichiro Matsumoto Recently, some experimental results showing the existence of nanobubbles which are bubbles of tens or hundreds of nanometer diameter in water have been reported. However, it is generally considered that such tiny bubbles are inherentlyunstable because of a strong effect of surface tension. Therefore, the stability of the nanobubbles are regarded as an unresolved problem. In order to investigate the stability mechanism, our approach is based on a microscopic standpoint. Especially, we introduced the density functional theory of classical systems to take into account the microscopic information on gas-- liquid interface and to execute the stability analyses in the microscopic system. The density functional approach has many advantages in the stability analysis of the two-phase microscopic system compared with the molecular dynamics approach. First, natural external conditions such as the grand canonical ensemble can be imposed. Second, the free energy evaluation which is required in the stability analyses can be performed directly. In this study, we focused on the effect of impurities in water on the stability of the nanobubbles. In the experimental system, electrolytes or surfactants are usually used in the literature, and we consider that the effect of the impurities is crucial. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y34.00002: Surface X-ray Scattering Observation of High-Density CO Monolayers on Pt(111) A. Menzel, Y.V. Tolmachev, V. Komanicky, A.V. Tkachuk, Y.S. Chu, H. You We report on surface x-ray scattering studies of high-density long-range ordered CO adsorbates on Pt(111) in equilibrium with gas phase at near-atmospheric CO partial pressures. We adsorbed CO with an electrochemical method and transferred the sample into a controlled gas atmosphere. At CO partial pressure $P_{\mathrm{CO}} \sim 1\,\mathrm{atm}$ we found long-range ordered $\left(2\! \times\!2\right)$-3CO, at $P_{\mathrm{CO}} < 0.5\,\mathrm{atm}$ we found $(\sqrt{19}\!\times\!\sqrt{19})\!R23.4^{\circ}$-13CO. These structures do not depend on the electrochemically controlled initial coverage, but are only a function of CO partial pressure and temperature. Based on our x-ray studies, we propose a phase diagram of CO monolayers on Pt(111) under near atmospheric pressure of CO. We will discuss the parallel behavior of CO monolayers in high-pressure atmospheres and those in electrochemical environment. However, there is disagreement between our results and those of recent studies of CO adsorbed from the gas phase, where $(\sqrt{19}\!\times\!\sqrt{19})$ has been reported for $P_{\mathrm{CO}}=1\,\mathrm{atm}$ and room temperature [Longwitz \emph{et al.}\ JPCB \textbf{108} 14497 (2004)]. Similar differences have been reported for studies in UHV when comparing gas adsorbed CO structures and electrochemically prepared structures studied \emph{in vacuo} [Zurawski \emph{et al.}\ JPC \textbf{94} 2076 (1990)]. We will address plausible causes for these discrepancies. [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y34.00003: Probing and modeling orbital-specific contributions to chemisorption using DFT Sara E. Mason, Ilya Grinberg, Andrew M. Rappe We present a DFT study of atomic and molecular chemisorption spanning a variety of transition metals, facets and adsorption geometries. We cast our results in a physical model for chemisorption that takes into account which metal and adsorbate orbitals interact to form chemisorption bonds. We use DFT chemisorption energies, orbital overlaps and changes in charge density induced by adsorption to track how the metal and adsorbate states interact as a function of metal identity, surface facet, adsorption geometry and strain. We introduce strain to the adsorption systems as a probe that causes relatively small, but geometrically specific, changes to electronic structure of the metal. By taking into account orbital specific contributions to chemisrotpion, our model is able to reproduce subtle differences in chemisorption present in our DFT results and offers a means for predicting adsorption energies on different metal surfaces at different sites and in different states of strain. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y34.00004: Electronic structure and bonding properties of potassium (K) on graphite under external electric field. Alejandro Tapia, Romeo de Coss, Gabriel Canto The effect of an external electric field on the potassium (K) adsorption on the graphite surface, are studied by means of first-principles total-energy calculations. The results were obtained with the pseudopotentials LCAO method (SIESTA code) and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. The structural parameters, bonding properties, and electronic structure of the K-graphite system are studied in the triangular $(2\times 2)$ overlayer phase as a function of the external electric field magnitude. We find an important change in the K-graphite bonding as a consequence of the charge transfer from the adatom towards the substrate induced by the electric field. The results are discussed in the light of the experimental observed difussion of K into graphite induced by external electric fields. This work was supported by Consejo Nacional de Ciencia y Tecnolog\'ia (CONACYT, M\'exico) under Grants No. 43830-F and No. 44831-F. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y34.00005: Chemical properties of Au nanoparticles on oxides Ruqian Wu Using the density functional DMol, VASP and FLAPW approaches, we studied the electronic and chemical properties of Au nano-particles on oxide substrates with O vacancies. Au clusters of more than 5 atoms are found to be unstable on MgO(001). Many configurations are explored for Au clusters on TiO$_{2}$(110) and SiO$_{2}$(0001). We found interesting size and shape dependence of their chemical properties, characterized by the density of states, HUMO/LUMO features and core level shifts. For substrates, we found that even SiO$_{2}$ monolayer displays a sizable gap on Mo(110). [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y34.00006: Quantum Delocalization and Hydrogen Adsorption on Pd (111) Surface Seong-Gon Kim, Sungho Kim, Steven C. Erwin The quantum delocalization of hydrogen atoms near the hydrogen atom vacancies on a palladium (111) surface is studied using density functional theory (DFT). Our quantum delocalization model elucidates the puzzle presented by a recent experiment [T.~Mitsui, \textit{et al}, Nature \textbf{422}, 705 (2003)]. In this experiment, Salmeron and his coworkers reported, contrary to conventional belief, that two-vacancy sites are inactive and that aggregates of three or more hydrogen vacancies are required for efficient hydrogen molecule dissociation and adsorption on a palladium (111) surface. Our total energy calculation shows that one or two hydrogen atoms are delocalized over three available adsorption sites. We found that delocalized vacancies provide a very compelling mechanism to explain the ineffectiveness of two-vacancy sites. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y34.00007: Density functional theory study of GaSb(100) surface reconstruction Jeffery Houze, Sungho Kim, Seong-Gon Kim We use density functional theory to study the (100) surface of GaSb semiconductors. 2x2 reconstructions of the Ga and Sb terminating surfaces revealed the following patterns. The Ga terminating surface exhibits a pattern alternating between dimerization and buckling. The Sb terminating surface exhibits only dimerization with each pair alternating in height. Simulations of 2x3 reconstructions will be presented to indicate possibility of more complex patterns. Surface and reconstruction energies will be given in support of number of fixed layers used for simulating bulk, and number of free layers needed to adequately simulate the reconstructions. Charge density plots will also be presented to explain interaction between surface atoms. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y34.00008: Hydrogen Adsorption on Stepped Surfaces of Metal Nickel Huiqiu Deng, Haixia Xiao, Wangyu Hu The chemisorption of gas molecules on transition-metals' surfaces has been an attractive field due to its importance in understanding the mechanisms of catalytic reactions. In the present paper the adsorption of hydrogen on nickel stepped surfaces (210), (211), (311), (410), (511), (977) and the low-index surfaces are studied with the embedded-atom model (EAM) many-body potentials which are based on density functional theory. The stable adsorption sites of hydrogen atoms are determined by the adsorption energies. The calculated results show that there exist more active adsorption sites near the steps. It is found that the stepped surfaces affect the adsorption properties of hydrogen seriously. The dissociative adsorption pathways of hydrogen molecules on the different nickel surfaces are also investigated. The results calculated in the present paper are in good agreement with the available experiment data and other theoretical values. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y34.00009: Adsorption Isotherm studies of Methyl Bromide adsorbed on Magnesium Oxide Teresa Burns, Michael Sprung, John Larese Understanding the interaction of polar molecules with ionic surfaces is technologically very important. Using high precision, volumetric adsorption isotherms the layering properties of methyl bromide on the MgO(100) surface were examined between 164 K and 179 K. Methyl bromide (Triple point = 179.49K) is found to exhibit two layering transitions within this temperature interval. Thermodynamic quantities derived from this study including the layering transition temperatures, the 2D compressibility, layer enthalpy and entropy of adsorption, and the isosteric enthalpy of adsorption will be presented. Comparisons with the adsorption properties of methyl chloride and methyl iodide will also be included. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y34.00010: The Role of the Ferroelectric Nature of a Substrate on its Ability to absorb Carbon Dioxide E. Ramos-Moore, Jurgen Baier, A. L. Cabrera The CO$_{2}$ adsorption on potassium niobate (KNbO$_{3})$ and potassium tantalate (KTaO$_{3})$ was studied in order to compared a ferroelectric and a parelectric surface towards CO$_{2}$ adsorption. Both oxides has Perovskite structure but when the niobate is ferroelectric at room temperature the tantalate is not. Characterization of the oxide powders was made by XRD, SEM and Micro-Raman spectroscopy. The adsorption of carbon dioxide was performed by TPD using the same system were BET surface area was obtained. TPD results will be displayed and discuss in the presentation. [Preview Abstract] |
Friday, March 25, 2005 1:15PM - 1:27PM |
Y34.00011: Vibrational Spectroscopy of Alcohol/ Silica Interfaces Weitao Liu, Luning Zhang, Y. Ron Shen Sum-frequency vibrational spectroscopy was used to probe alcohol/silica interfaces in order to study how methanol, ethanol, and 1- and 2-propanol molecules orient at the interfaces. It was found that in the CH stretch range, the symmetric CH$_{3}$ peaks of all alcohols decreased comparing with the air/liquid interface, while the spectra of ethanol and propanol exhibited prominent antisymmetric CH$_{3}$ peaks. The results can be understood by knowing that the first interfacial monolayer of alcohol molecules must adsorb with their OH terminal on the hydrophilic silica and the CH$_{3}$ terminal toward liquid, but the adjacent less ordered alcohol monolayer should have the reverse orientation. Because the CH$_{3}$ groups of the two monolayers are oppositely oriented, their symmetric stretch modes tend to cancel each other in SFG, but the antisymmetric stretch modes generally do not. In the case of methanol, the antisymmetric stretch is intrinsically weak in the sum-frequency spectra, making the overall spectra very weak. [Preview Abstract] |
Friday, March 25, 2005 1:27PM - 1:39PM |
Y34.00012: From cluster to bulk: Size dependent energetics of silica and silica-water interaction Yao He, Chao Cao, Yin-Xia Wan, Hai-Ping Cheng We present our computational studies on the energetics of clusters that consist of H2O and SiO2 using first-principles Born-Oppenheimer molecular dynamics method. Cohesive energy and hydration energy of both pure (or dry) and hydroxylated (or wet) ring-structured clusters are investigated as functions of system size. We have found clear trends in both energy values as the cluster size increases. Energetics of a small silica rod that contains 108 atoms is also obtained as a middle reference point for size evolution. The interaction of the nano-rod with water molecular has been investigated thoroughly at various sites. Results from cluster calculations are compared with one from bulk quartz and cristobalite calculations. (Acknowledgement: This work is supported by NSF/ITR grant). [Preview Abstract] |
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Y34.00013: Nonlinear Optical Probe of Buried Interface of Organic Thin Films Susan Dounce, Minchul Yang, Tom Rockey, Hai-Lung Dai Nanometer-thick thin films of small organic molecules have found many recent technology applications in organic semiconducting devices where the interfacial layer structure presumably greatly affects the film electrical properties. In this presentation we show that by using a combination of nonlinear optical and surface science techniques we can determine the intrinsic structure within the molecular films including the orientation and alignment of the molecules at the film/substrate interface. It is demonstrated by using optical second harmonic generation that the pyridine molecules in the interfacial layer underneath the film are found to align along the [11bar0] direction of the Ag(110) surface with a small tilt angle (11 degrees) from the surface norm. This interfacial ordering is found to have a notable effect in inducing crystallization at the heterogeneous boundary of the amorphous molecular film. [Preview Abstract] |
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